Spot

Spot is a lightweight dependency injection (DI) framework for Dart and Flutter applications. Technically implemented as a service locator pattern, Spot provides a minimal yet powerful API for managing dependencies with support for singletons, factories, async initialization, and scoped containers.

Features

• 🎯 Simple API - Register and resolve dependencies with minimal boilerplate
• 🏭 Factory & Singleton Support - Choose the right lifecycle for each dependency
• ⚡ Async Initialization - Handle services that require async setup
• 🔍 Named Instances - Register multiple implementations of the same type
• 🧪 Scoped Containers - Isolate dependencies for testing or feature modules
• ♻️ Lifecycle Management - Automatic cleanup via SpotDisposable interface
• 🔒 Type Safety - Full compile-time type checking with generics
• 🚫 Circular Dependency Detection - Clear error messages when things go wrong
• 📦 Zero Dependencies - No external runtime dependencies

Installation

Add this to your pubspec.yaml:

dependencies:
  spot: ^1.0.0

Then run:

dart pub get

Quick Start

Basic Usage

import 'package:spot/spot.dart';

// 1. Define your interfaces and implementations
abstract class ILogger {
  void log(String message);
}

class ConsoleLogger implements ILogger {
  @override
  void log(String message) => print('[LOG] $message');
}

// 2. Register dependencies
void main() {
  Spot.registerSingle<ILogger, ConsoleLogger>((get) => ConsoleLogger());
  
  // 3. Resolve and use
  final logger = spot<ILogger>();
  logger.log('Hello, Spot!');
}

Registration Patterns

Singleton - One instance shared across the app:

Spot.registerSingle<ISettings, AppSettings>((get) => AppSettings());

final settings1 = spot<ISettings>();
final settings2 = spot<ISettings>();
print(identical(settings1, settings2)); // true

Factory - New instance on each resolution:

Spot.registerFactory<IRepository, UserRepository>(
  (get) => UserRepository(),
);

final repo1 = spot<IRepository>();
final repo2 = spot<IRepository>();
print(identical(repo1, repo2)); // false

Async Singleton - For services requiring async initialization:

Spot.registerAsync<Database, AppDatabase>((get) async {
  final db = AppDatabase();
  await db.initialize();
  return db;
});

// Must use spotAsync for async singletons
final db = await spotAsync<Database>();

Dependency Injection

Services can depend on other services using the get parameter:

// Register dependencies in order
Spot.registerSingle<ILogger, ConsoleLogger>((get) => ConsoleLogger());

Spot.registerSingle<IApiClient, ApiClient>((get) {
  return ApiClient(
    logger: get<ILogger>(),  // Inject dependencies
  );
});

// Spot handles the dependency graph
final apiClient = spot<IApiClient>();

Bulk Registration

Use the init helper for cleaner registration:

Spot.init((factory, single) {
  // Register singletons
  single<ILogger, ConsoleLogger>((get) => ConsoleLogger());
  single<ISettings, AppSettings>((get) => AppSettings());
  
  // Register factories
  factory<IRepository, UserRepository>(
    (get) => UserRepository(get<ILogger>()),
  );
});

Named Instances

Register multiple implementations of the same type:

Spot.registerSingle<HttpClient, PublicHttpClient>(
  (get) => PublicHttpClient(),
  name: 'public',
);

Spot.registerSingle<HttpClient, AuthenticatedHttpClient>(
  (get) => AuthenticatedHttpClient(),
  name: 'authenticated',
);

// Resolve by name
final publicClient = spot<HttpClient>(name: 'public');
final authClient = spot<HttpClient>(name: 'authenticated');

Lifecycle Management

Implement SpotDisposable for automatic cleanup:

class DatabaseService implements SpotDisposable {
  late Database _db;
  
  @override
  void dispose() {
    _db.close();
  }
}

Spot.registerSingle<DatabaseService, DatabaseService>(
  (get) => DatabaseService(),
);

// Cleanup specific service
Spot.dispose<DatabaseService>();  // Calls dispose() automatically

// Or cleanup everything on app shutdown
Spot.disposeAll();

Scoped Containers

Create isolated dependency scopes for testing or feature modules:

// Global dependencies
Spot.registerSingle<ISettings, AppSettings>((get) => AppSettings());

// Create test scope
final testScope = Spot.createScope();
testScope.registerSingle<ISettings, MockSettings>(
  (get) => MockSettings(),
);

// Each scope has its own version
final prodSettings = spot<ISettings>();        // Gets AppSettings
final testSettings = testScope.spot<ISettings>();  // Gets MockSettings

// Cleanup test scope (doesn't affect global)
testScope.dispose();

Nested Scopes

Scopes can inherit from parent scopes:

final parentScope = Spot.createScope();
final childScope = parentScope.createChild();

// Child falls back to parent for missing dependencies
parentScope.registerSingle<ILogger, ConsoleLogger>(
  (get) => ConsoleLogger(),
);

final logger = childScope.spot<ILogger>();  // Gets from parent

Advanced Usage

Checking Registration

if (Spot.isRegistered<ILogger>()) {
  print('Logger is registered');
}

if (Spot.isRegistered<HttpClient>(name: 'public')) {
  print('Public HTTP client is registered');
}

Debugging

// Enable verbose logging
Spot.logging = true;

// Print all registered types
Spot.printRegistry();
// Output:
// === Spot Registry (3 types) ===
//   ILogger -> ConsoleLogger [singleton] (initialized)
//   ISettings -> AppSettings [singleton]
//   IRepository -> UserRepository [factory]
// ==================================================

Error Handling

Spot provides clear error messages:

// Unregistered type
try {
  final service = spot<UnregisteredService>();
} catch (e) {
  // SpotException: Type UnregisteredService is not registered in Spot container.
  // Registered types: ILogger, ISettings
}

// Circular dependency
Spot.registerSingle<ServiceA, ServiceA>(
  (get) => ServiceA(get<ServiceB>()),
);
Spot.registerSingle<ServiceB, ServiceB>(
  (get) => ServiceB(get<ServiceA>()),
);

try {
  spot<ServiceA>();
} catch (e) {
  // SpotException: Circular dependency detected: ServiceA -> ServiceB -> ServiceA
}

Testing

Spot makes testing easy with scoped containers:

import 'package:test/test.dart';
import 'package:spot/spot.dart';

void main() {
  // Setup production dependencies once
  setUpAll(() {
    Spot.registerSingle<IApiClient, ApiClient>((get) => ApiClient());
  });
  
  // Clean up after each test
  tearDown(() {
    Spot.disposeAll();
  });
  
  test('with mocked dependencies', () {
    // Create isolated test scope
    final testScope = Spot.createScope();
    testScope.registerSingle<IApiClient, MockApiClient>(
      (get) => MockApiClient(),
    );
    
    // Test with mock
    final apiClient = testScope.spot<IApiClient>();
    expect(apiClient, isA<MockApiClient>());
    
    // Cleanup
    testScope.dispose();
  });
}

Service Locator Pattern

Spot is technically a service locator pattern rather than pure dependency injection. This means:

What it is:

• A centralized registry where services can be registered and retrieved
• Dependencies are resolved at runtime using spot<T>()
• Simple, straightforward, and easy to understand

What it's not:

• Not constructor injection (dependencies aren't automatically injected)
• Not compile-time dependency resolution
• Not a full-featured DI container like Angular's injector

Why this is fine for Dart/Flutter:

• Dart doesn't have built-in reflection for constructor injection
• Service locator pattern is lightweight and performant
• Most Flutter apps use similar patterns (GetIt, Provider, etc.)
• You get the benefits of DI (loose coupling, testability) with minimal overhead

API Reference

Registration Methods

Method	Description
registerSingle<T, R>(locator)	Register a singleton (lazy initialization)
registerFactory<T, R>(locator)	Register a factory (new instance each time)
registerAsync<T, R>(locator)	Register async singleton
init(initializer)	Bulk registration helper

Resolution Methods

Method	Description
spot<T>({name})	Resolve dependency synchronously
spotAsync<T>({name})	Resolve async singleton
Spot.spot<T>({name})	Static method (same as global spot)
Spot.spotAsync<T>({name})	Static method (same as global spotAsync)

Lifecycle Methods

Method	Description
dispose<T>({name})	Dispose specific service
disposeAll()	Dispose all services

Utility Methods

Method	Description
isRegistered<T>({name})	Check if type is registered
printRegistry()	Print all registered types (debugging)
createScope()	Create scoped container

Container Methods

Method	Description
container.registerSingle<T, R>(locator)	Register in scope
container.registerFactory<T, R>(locator)	Register factory in scope
container.registerAsync<T, R>(locator)	Register async in scope
container.spot<T>({name})	Resolve from scope
container.spotAsync<T>({name})	Resolve async from scope
container.createChild()	Create nested scope
container.dispose()	Dispose scope

Examples

See the example directory for complete examples:

• spot_example.dart - Basic usage patterns
• spot_flutter_example.dart - Flutter integration

Development

Git Hooks

This repository includes Git hooks to ensure code quality:

• pre-push: Runs dart analyze and dart test before pushing any branch

To install the hooks after cloning:

./hooks/install.sh

The hooks will automatically prevent pushes if analysis or tests fail.

Contributing

Contributions are welcome! Please feel free to submit issues or pull requests.

Before submitting a PR, make sure to:

1. Install the Git hooks: ./hooks/install.sh
2. Run dart analyze to check for issues
3. Run dart test to ensure all tests pass
4. Run dart format . to format your code

License

This project is licensed under the MIT License - see the LICENSE file for details.